A wide variety of cordless power tools are available that may be utilized in different applications such as construction applications, fire and rescue applications, etc. Some examples of cordless power tools include, but are not limited to, cordless drills, cordless circular saws, cordless reciprocating saws, cordless sanders, cordless screwdrivers, and flashlights. Cordless power tools may utilize a rechargeable battery pack for providing power to operate the tool. The rechargeable battery pack may be readily removed from the cordless power tool and coupled to an external battery charger for charging purposes.
The battery pack may include one or more battery cells. The battery cells may be nickel-cadmium or nickel-metal hydride cells. Other types of cells such as lithium ion cells may also be utilized. Since lithium ion cells may suffer from degraded performance or may become hazardous if charged above a specified level or discharged below a specified level, monitoring circuitry may be incorporated into the battery pack to monitor cell voltage levels. The monitoring circuitry may also monitor current levels flowing into or out of the battery pack. A sense resistor in the battery pack may be utilized to provide a signal to the monitoring circuitry representative of current flow. The sense resistor contributes an additional expense and contributes to power losses. The power losses are exacerbated with increasing current levels utilized in high current cordless power tools such as a drill or circular saw.
Accordingly, there is a need to remove the sense resistor and provide a weak link protection system for a cordless power tool.